The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
A wind turbine is a device that can be used to convert kinetic energy (from the wind) into mechanical energy, for example by using wind to rotate turbine rotor blades. An example of a classic wind turbine is a horizontal axis wind turbine comprising an electrical generator in a horizontal central hub with turbine rotor blades attached. The turbine rotor delivers torque energy to the generator hub which converts this to electrical energy via an electrical generator. However, a classical wind turbine that is not augmented with a diffuser is limited in the amount of energy it can convert from wind by Betz' law, which states that for a classical wind turbine, in which the airflow is axial and free streaming, no more than 59.3% of kinetic energy from wind can be converted to mechanical energy.
Wind turbine diffusers may be used to increase the velocity of the air entering the turbine's rotor plane, thus increasing the power output and efficiency of any given turbine. Conventional diffusers require a large expansion in exit area of the wind turbine in order to induce a relatively modest increase in wind speed (and hence, power output and efficiency). This increased exit area reduces the air pressure behind the turbine and leads to air being accelerated over the turbine rotor blades. However, expanding the exit area of the wind turbine generally results in the use of large, uneconomic and highly drag-loaded cowlings around the turbine. US 2005/0002783 A1 describes a diffuser augmented wind-turbine which has an expanded exit area.
Another example of a conventional wind turbine diffuser is one that uses a mixer-ejector system to draw high energy air into the rear section of the diffuser. In theory, this system decreases the air pressure behind the wind turbine and therefore draws more air through the turbine rotor plane, which increases the power output and efficiency. However, injection-mixer systems may not provide as significant an increase in the velocity of the air passing through the turbine in practice and therefore may not provide a significant increase in power output efficiency. WO 2010/036678 A1 describes a mixer-ejector (MEWT) wind turbine.
Wind turbine rotor cowlings that are attached to and rotate with the turbine rotor blade tips may be used to reduce the noise and disruption caused by blade tip vortices. However, they are not designed to reduce the pressure behind the turbine plane and therefore accelerate the air through the turbine plane thereby increasing power output and efficiency. WO 2004/083631 A2 describes a wind turbine with a rotatable cowling which is designed to eliminate blade tip vortices. WO 2010/131052 describes a static wind turbine cowling which has angled slots in front of the turbine plane which induce rotational flow acting against the post-rotor wake. This acts like an anti-swirl device, re-aligning the flow.